Dye-sensitized solar cell and method for preventing elution of catalyst from catalyst electrode

ABSTRACT

Providing a dye-sensitized solar cell having high durability and thermal resistance, and preventing elution of a platinum group catalyst from a catalytic electrode: by surface-treating the catalytic electrode with (a) a specific sulfur material having a molecular weight of 32 to 10,000 containing a sulfur atom having an oxidation number of −2 to 0, (b) another specific sulfur material containing no sulfur atom having an oxidation number of −2 to 0, but containing a sulfur atom having an oxidation number of +1 to +4 [with the proviso that the sulfur material (b) is such a material that a surface of the surface-treated catalyst electrode has a photoelectron peak within a binding energy range of 161 to 165 eV in an X-ray photoelectron spectrum], or (c) a mixture of the sulfur materials (a) and (b); and/or by adding the sulfur material into the electrolyte layer.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/812,478 filed on Jan. 25, 2013, which claims priority to PCTApplication No. PCT/JP2011/004120, filed on Jul. 21, 2011, which claimspriority to Japanese Application No. JP 2010-168415, filed on Jul. 27,2010, the contents of which are incorporated herein by specificreference.

TECHNICAL FIELD

The present invention relates to a dye-sensitized solar cell having highdurability and thermal resistance, particularly to a technology forpreventing elution of a catalyst from a catalyst electrode.

BACKGROUND ART

Typical examples of a catalyst electrode (counter electrode) used in adye-sensitized solar cell are platinum electrodes in whichchloroplatinic acid is applied to and heated on an electrode substrate,and in which platinum is vapor-deposited or electro-deposited on anelectrode substrate. In addition, an I₂/I₃ ⁻ based redox system, whichdemonstrates well-balanced performances, is a typical example of anelectrolyte used in a dye-sensitized solar cell.

However, a problem has been reported that platinum serving as a catalystis dissolved in an electrolyte solution having an I₂/I₃ ⁻ based redoxsystem or similar system (see NPL 1, paragraph 0004 and so forth of PTL1).

Several techniques for addressing such a problem are reported.

For example, PTL 1 discloses an electrode including a π-conjugatedconductive polymer layer formed as a corrosion-resistant conductivecoating material on an intermediate layer made of a platinum group metallayer and/or an oxide layer thereof formed on a substrate. As thecorrosion-resistant conductive coating material, polypyrrole,polyaniline, polythiophene, or derivatives thereof are exemplified.

Moreover, PTL 2 filed by the same applicant as PTL 1 above discloses, asa highly corrosion-resistant catalyst electrode, a catalyst electrodehaving a corrosion-resistant conductive layer on a metal layer on acatalyst electrode. As the material for forming the corrosion-resistantconductive layer, various metal oxides, metal nitrides, and metalborides are exemplified.

Further, PTL 3 mainly targets a fuel cell, and does not mention adye-sensitized type solar cell, but states that elution of platinum canbe prevented by supplying a reactive gas containing a heterocycliccompound, for example, pyridines such as bipyridine, terpyridine, andphenanthroline to an electrochemical cell to bring the reactive gas intocontact with a platinum-containing electrode catalyst layer in theelectrochemical cell.

Furthermore, PTL 4 can be cited as an example similar to PTL 1 above interms of materials for an electrode, even though PTL 4 does notparticularly mention the durability and thermal resistance of anelectrode. PTL 4 discloses an electrode in which a conductive layercontaining platinum particles and a conductive binder for binding theplatinum particles is formed on a substrate and the voids incommunicating with a surface of the conductive layer are formed amongthe platinum particles, especially in order to surely obtain a largesurface area for the electrode surface. As the conductive binder,polythiophene such as poly(3,4-ethylenedioxythiophene), polypyrrole, andpolyaniline are exemplified.

CITATION LIST Patent Literatures

-   PTL 1: Japanese Patent Laid-Open No. 2008-266744-   PTL 2: Japanese Patent Laid-Open No. 2006-318770-   PTL 3: Japanese Patent Laid-Open No. 2009-016273-   PTL 4: Japanese Patent Laid-Open No. 2006-147412

Non Patent Literatures

-   NPL 1: E. Olsten, et al., Solar Energy Mater. Solar Cells, 63, 267    (2000)-   NPL 2: C. Zhang et al., J. Phys. Chem. B 2006, 110, 12485-12489-   NPL 3: N. Kopidakis et al., J. Phys. Chem. C 2009, 113, 21779-21783-   NPL 4: R&D Review of Toyota CRDL, vol. 35, No. 4, pp. 43-50 (2000,    12)-   NPL 5: M. Graetzel et al., Chem. Mater., 2004, 16, 2694-2696

SUMMARY OF INVENTION

An object of the present invention is to improve the durability andthermal resistance of a dye-sensitized solar cell by preventing elutionof a catalyst from a catalyst electrode, and more generally to provide anew method for improving the durability and thermal resistance of acatalyst electrode.

1. The first embodiment of the present invention is a dye-sensitizedsolar cell characterized in that the dye-sensitized solar cell is adye-sensitized type solar cell comprising: a semiconductor electrodecontaining a photo-sensitizing dye; an electrolyte layer containingchemical species as a redox couple; and a counter electrode disposedopposite to the semiconductor electrode with the electrolyte layerinterposed therebetween,

the counter electrode is a catalyst electrode containing a platinumgroup, and

the catalyst electrode is surface-treated with any one of sulfurmaterials:

-   -   (a) at least one sulfur material having a molecular weight of 32        to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c) a mixture of the sulfur materials (a) and (b),

with the proviso that the sulfur material (b) is such a material that asurface of the catalyst electrode surface-treated with the sulfurmaterial (b) has a photoelectron peak within a binding energy range of161 to 165 eV in an X-ray photoelectron spectrum.

2. The second embodiment of the present invention is a dye-sensitizedsolar cell characterized in that the dye-sensitized solar cell is adye-sensitized type solar cell comprising: a semiconductor electrodecontaining a photo-sensitizing dye; an electrolyte layer containingchemical species as a redox couple; and a counter electrode disposedopposite to the semiconductor electrode with the electrolyte layerinterposed therebetween,

the counter electrode is a catalyst electrode containing a platinumgroup, and

the electrolyte layer contains any one of sulfur materials:

-   -   (a) at least one sulfur material having a molecular weight of 32        to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c) a mixture of the sulfur materials (a) and (b), with the        proviso that:

the sulfur compound (b) is such a material that when the catalystelectrode is surface-treated with the sulfur material (b), the treatedsurface of the catalyst electrode has a photoelectron peak within abinding energy range of 161 to 165 eV in an X-ray photoelectronspectrum;

when the sulfur material (a) is guanidinium thiocyanate, the guanidiniumthiocyanate satisfies a requirement of having a concentration of theguanidinium thiocyanate is less than 0.1 M in the electrolyte layer; and

when the sulfur material (a) is 1-ethyl-3-methylimidazolium thiocyanate,a volume percent of the 1-ethyl-3-methylimidazolium thiocyanate is lessthan 35% by volume in a solvent of an electrolyte solution,

where the volume percent of the 1-ethyl-3-methylimidazolium thiocyanateis calculated based on a sum of each volume of a main solvent and one ormore cosolvents where

the main solvent is a liquid component present in a largest amount(based on the volume of the component present alone) among components ina liquid state at standard ambient temperature and pressure (25° C., 1atm) of the electrolyte layer, and

the one or more cosolvents are 1-ethyl-3-methylimidazolium thiocyanate(ionic liquid) and one or more optional liquid components, each of theoptional liquid components having a one-fifth volume or more of the mainsolvent (based on the volumes of the components each present alone).

3. The third embodiment of the present invention is a dye-sensitizedsolar cell characterized in that the dye-sensitized solar cell is adye-sensitized type solar cell comprising: a semiconductor electrodecontaining a photo-sensitizing dye; an electrolyte layer containingchemical species as a redox couple; and a counter electrode disposedopposite to the semiconductor electrode with the electrolyte layerinterposed therebetween,

the counter electrode is a catalyst electrode containing a platinumgroup,

the catalyst electrode is surface-treated with any one of first sulfurmaterials:

-   -   (a1) at least one sulfur material having a molecular weight of        32 to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b1) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c1) a mixture of the sulfur materials (a1) and (b1), and

the electrolyte layer contains any one of second sulfur materials:

-   -   (a2) at least one sulfur material having a molecular weight of        32 to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b2) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c2) a mixture of the sulfur materials (a2) and (b2),

with the proviso that the sulfur material (b1) among the first sulfurmaterials is such a material that a surface of the catalyst electrodesurface-treated with the sulfur material (b1) has a photoelectron peakwithin a binding energy range of 161 to 165 eV in an X-ray photoelectronspectrum; and

the sulfur material (b2) among the second sulfur materials is such amaterial that when the catalyst electrode is surface-treated with thesulfur material (b2), the treated surface of the catalyst electrode hasa photoelectron peak within a binding energy range of 161 to 165 eV inan X-ray photoelectron spectrum.

4. The fourth embodiment of the present invention is a method forpreventing elution of a platinum group catalyst from a catalystelectrode having the platinum group catalyst into an electrolyte layerin contact with the catalyst electrode, where

the electrolyte layer contains chemical species as a redox couple,

the catalyst electrode is surface-treated with any one of first sulfurmaterials:

-   -   (a1) at least one sulfur material having a molecular weight of        32 to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b1) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c1) a mixture of the sulfur materials (a1) and (b1), and/or

the electrolyte layer contains any one of second sulfur materials:

-   -   (a2) at least one sulfur material having a molecular weight of        32 to 10,000 selected from elemental sulfur, inorganic sulfur        compounds each containing at least one sulfur atom having an        oxidation number of −2 to 0, and organic sulfur compounds each        containing at least one sulfur atom having an oxidation number        of −2 to 0;    -   (b2) at least one sulfur material containing no sulfur atom        having an oxidation number of −2 to 0 selected from inorganic        sulfur compounds each containing at least one sulfur atom having        an oxidation number of +1 to +4 and organic sulfur compounds        each containing at least one sulfur atom having an oxidation        number of +1 to +4; and    -   (c2) a mixture of the sulfur materials (a2) and (b2),

with the proviso that the sulfur material (b1) among the first sulfurmaterials is such a material that a surface of the catalyst electrodesurface-treated with the sulfur material (b1) has a photoelectron peakwithin a binding energy range of 161 to 165 eV in an X-ray photoelectronspectrum; and

the sulfur material (b2) is such a material that when the catalystelectrode is surface-treated with the sulfur material (b2), the treatedsurface of the catalyst electrode has a photoelectron peak within abinding energy range of 161 to 165 eV in an X-ray photoelectronspectrum.

The first embodiment of the present invention makes it possible toprevent elution of a catalyst from the catalyst electrode, accordinglyenabling production of a solar cell having high durability and thermalresistance. In addition, at least in a preferred mode of the presentinvention, the catalytic activity of the catalyst electrode can also beenhanced, thereby improving not only the durability and the thermalresistance but also the photoelectric conversion efficiency of the solarcell.

Moreover, in the second embodiment of the present invention, the meansis merely adding a particular sulfur material into the electrolytesolution, and accordingly there are advantages of being very simple, lowcost, and capable of improving the durability and the thermal resistanceof the catalyst electrode and a cell using the same. Additionally, itcan be expected that a sulfur coated film formed on the surface of thecatalyst electrode lasts long, the sulfur coated film being believed tocontribute to the durability and the thermal resistance.

Further, the third embodiment of the present invention can be expectedto have the merits of both of the first embodiment and the secondembodiment of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sodium sulfide;

FIG. 1B is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with potassium thiocyanate;

FIG. 2A is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with thiourea;

FIG. 2B is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with elemental sulfur;

FIG. 3A is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sodium thiosulfate;

FIG. 3B is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with dimethyl sulfoxide;

FIG. 4A is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sodium dithionite;

FIG. 4B is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sulfolane;

FIG. 5A is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sodium sulfite; and

FIG. 5B is a graph showing a photoelectron peak within a S2p region inan X-ray photoelectron spectrum of a surface of a platinum catalystelectrode surface-treated with sodium sulfate.

DESCRIPTION OF EMBODIMENTS 1. First Embodiment of the Present Invention

In the first embodiment of the present invention, a dye-sensitized typesolar cell is characterized in that a catalyst electrode issurface-treated with a particular sulfur material.

In the present invention, an effect of suppressing elution of a catalystand at least in a preferred mode, an effect of improving the catalyticactivity are demonstrated. Although the mechanism is not known indetail, it is speculated that by the contact with such a particularsulfur material, a coated film of a single molecular or single atomiclayer containing a sulfur atom in a low oxidation state (having anoxidation number of −2 to 0) is formed on a surface of the metalcatalyst, and the sulfur atom creates an reducing environment on themetal catalyst surface. Specifically, the reducing environmentpresumably suppresses oxidation and dissolution of the catalyst by anoxidizing substance (such as I₂) in an electrolyte, and at least in apreferred mode, the bonding or interaction with the sulfur atom changesthe electronic state of the metal catalyst, and may also enhance thecatalytic activity.

1) Sulfur material

(1) In the present embodiment, the sulfur material refers to any one ofsulfur materials:

(a) at least one sulfur material having a molecular weight of 32 to10,000, more preferably 32 to 5,000, further preferably 32 to 1,000, andstill further preferably 32 to 500 selected from elemental sulfur,inorganic sulfur compounds each containing at least one sulfur atomhaving an oxidation number of −2 to 0, and organic sulfur compounds eachcontaining at least one sulfur atom having an oxidation number of −2 to0;

(b) at least one sulfur material containing no sulfur atom having anoxidation number of −2 to 0 selected from inorganic sulfur compoundseach containing at least one sulfur atom having an oxidation number of+1 to +4, more preferably having an oxidation number of +3 or +4, andorganic sulfur compounds each containing at least one sulfur atom havingan oxidation number of +1 to +4, more preferably +3 or +4; and

-   -   (c) a mixture of the sulfur materials (a) and (b).

Nevertheless, it is required that the sulfur material (b) be such amaterial that the surface of the catalyst electrode surface-treated withthe sulfur material (b) has a photoelectron peak within a binding energyrange of 161 to 165 eV in an X-ray photoelectron spectrum.

(2) In the present embodiment, the sulfur material (a) contains at leastone sulfur atom having an oxidation number of −2 to 0, while the sulfurmaterial (b) contains at least one sulfur atom having an oxidationnumber of +1 to +4 but does not contain sulfur atoms having an oxidationnumber of −2 to 0.

The oxidation number is an apparent oxidation state of an element; (i)the oxidation number of each atom in a covalent compound is theresulting formal charge number of the atom, when an electron pair sharedbetween two atoms is all assigned to the more electronegative atom, (ii)the oxidation number of a monatomic ion in an ionic compound is equal tothe charge number of the ion, and (iii) the oxidation number of an atomin an elemental form is always 0.

Note that the electronegativity refers to the tendency of an atom to beelectrically negative by attracting electrons through the bond. Forexample, according to the Pauling scale, the values of main elements areas follow:

Element (Electronegativity):

H (2.1), C (2.5), N (3.0), 0 (3.5), S (2.5).

(3) It is believed that a sulfur coated film in a low oxidation state(having an oxidation number of −2 to 0) is formed on the surface of thecatalyst electrode surface-treated with the sulfur material (a). Thiscan be confirmed by a photoelectron peak presumably due to sulfur in alow oxidation state (having an oxidation number of −2 to 0) presentwithin a S2p region (a photoelectron peak region of electrons in 2porbital of a sulfur atom) of 161 to 165 eV in the X-ray photoelectronspectrum of the surface of the catalyst electrode (see FIGS. 1A, 1B, 2A,2B, and 3A and NPL 4).

In contrast, it is believed that in the case of the sulfur material(b1), once the sulfur atom having an oxidation number of +1 to +4 isreduced on the surface of the catalyst electrode surface-treated withthe sulfur material, a sulfur coated film formed on the surface of thecatalyst electrode contains the sulfur atom converted to the same lowoxidation state (having an oxidation number of −2 to 0) as that of thesulfur material (a) above. This is speculated because similarly to thecase of the sulfur material (a), a photoelectron peak presumably due tosulfur in a low oxidation state (having an oxidation number of −2 to 0)is observed within a S2p region (a photoelectron peak region ofelectrons in 2p orbital of a sulfur atom) of 161 to 165 eV in an X-rayphotoelectron spectroscopy spectrum of a surface of a catalyst electrodetreated with sodium dithionite containing a sulfur atom having anoxidation number of +3 or sulfolane containing a sulfur atom having anoxidation number of +4 (see FIGS. 4A and 4B).

Since it is believed that the sulfur atom having an oxidation number of+1 to +4 undergoes a reduction process to be a sulfur atom in the lowoxidation state, the sulfur material (b) is required to be such amaterial that the surface of the catalyst electrode surface-treated withthe sulfur material (b) has a photoelectron peak within a binding energyrange of 161 to 165 eV in an X-ray photoelectron spectrum.

(4) Elemental sulfur is sulfur present in a free state, such as naturalsulfur. Note that since it is in the elemental form, the sulfur has anoxidation number of 0.

(5) Examples of the inorganic sulfur compounds as the sulfur material(a) include the following (i), (ii), and (iii).

(i) Metal sulfide salts, metal salts of hydrogen sulfide, ammoniumsulfide salts, primary to quaternary ammonium sulfide salts, ammoniumsalts of hydrogen sulfide, primary to quaternary ammonium salts ofhydrogen sulfide, and hydrogen sulfide.

The metal is preferably an alkali metal or alkaline earth metal.

Examples of primary to quaternary ammoniums include pyridinium,guanidinium, tetrapropylammonium, pyrrolidinium, piperidinium, and thelike.

More preferable is one represented by the following formula:[Chemical formula 1]M₂S  (I)(where two Ms are the same or different, and are each an alkali metal,ammonium, pyridinium, guanidinium, or hydrogen).

More specifically, sodium sulfide, hydrogen sulfide, and the like can beexemplified.

Note that the sulfur atom in this compound has an oxidation number of−2.

(ii) Thiosulfuric acid, and salts and esters thereof

Examples of a counter cation in the salts include alkali metals,alkaline earth metals, ammonium, and primary to quaternary ammoniums(pyridinium, guanidinium, tetrapropylammonium, pyrrolidinium,piperidinium, and the like). Although falling into the category oforganic compounds, examples of the esters here for convenience includeesters from alcohols having 1 to 40, more preferably 1 to 20, andfurther preferably 1 to 10 total carbon atoms which may have asubstituent.

More preferable is the following formula:

(where two Ms are the same or different, and are each an alkali metal,ammonium, pyridinium, guanidinium, or a hydrogen ion).

More specifically, sodium thiosulfate can be exemplified.

Note that one sulfur atom of the two sulfur atoms in this compound hasan oxidation number of +4, and the other sulfur atom has 0.

(iii) Carbon sulfide

Examples of carbon sulfide include carbon disulfide, dicarbontrisulfide, carbon monosulfide, and carbon sulfite. An example of sulfurnitride includes tetrasulfur tetranitride.

Sulfur atoms in carbon disulfide, dicarbon trisulfide, carbonmonosulfide, and carbon sulfite, all belonging to carbon sulfide have anoxidation number of 0.

(6) Examples of the organic sulfur compounds as the sulfur material (a)include the following (iv) to (vii).

(iv) Organic sulfur compounds having one or more sulfur functionalgroups selected from a thiol group [—SH] and salts thereof, ahydropolysulfide group [—(S)_(n)SH, where n is an integer of greaterthan or equal to 1] and salts thereof, a sulfide group [—S—], apolysulfide group [—(S)_(n)S—, where n is an integer of greater than orequal to 1], a thiocarbonyl group [—C(═S)—], a thioaldehyde group[—C(═S)H], a thiocarboxyl group [—C(═S)OH or —C(═O)SH] and saltsthereof, and esters, amides, imides, acid anhydrides, and acid halidesthereof, a dithiocarboxylic acid [—C(═S)SH] and salts and estersthereof, a thioacetal group, and a thioketal group.

More specifically, typical examples include those represented by thefollowing formulas.

where R¹, R², R³, and R⁴ are the same or different, and are each ahydrocarbon group having 1 to 40, more preferably 1 to 20, and furtherpreferably 1 to 10 total carbon atoms which may have a substituent.

Moreover, in the above formulas (V), (VI), (VII), (IX-1), (IX-2), and(X), R¹ and R² may be bonded together to form a ring.

Further, in the above formulas (XI), (XII-2), and (XII-3), R¹, R², andR³ may be bonded together to form a ring. For example, R¹ and R² may bebonded together to form a ring, or three of R¹, R², and R³ may be bondedtogether to form a fused bicyclic ring.

Furthermore, in the formula (XII), R¹, R², R³, and R⁴ may be bondedtogether to form a ring. For example, R¹ and R² may be bonded togetherto form a ring; R¹ and R may be bonded together, while R³ and R⁴ may bebonded together to form a bicyclic ring; R¹, R², and R³ may be bondedtogether to form a fused bicyclic ring; R¹, R², R³, and R⁴ may be bondedtogether to form a fused tricyclic ring.

More specifically, n-dodecanethiol, methionine, triazine trithiol asshown in the following formula (XIII) (2,4,6-trimercapto-s-triazine monosodium salt), and the like can be exemplified.

Note that the sulfur atoms in this compound have an oxidation number of−1 or 0.

(v) Thiocyanic acid, salts and esters thereof, and isothiocyanic acidesters.

More specifically, typical examples include those represented by thefollowing formula.[Chemical formula 5]R⁵SCN  (XIV)R⁶NCS  (XV)

where R⁵ and R⁶ are the same or different, and are each hydrogen, acounter cation, or a hydrocarbon group having 1 to 40, more preferably 1to 20, and further preferably 1 to 10 total carbon atoms which may havea substituent.

Examples of a counter cation of the thiocyanic acid salts include alkalimetal ions, alkaline earth metal ions, an ammonium ion, primary toquaternary ammonium ions, and the like.

More specifically, thiocyanic acid sodium salt, isothiocyanic acidcyclohexyl ester shown in the following formula, and the like can beexemplified.

Note that the sulfur atom in this compound has an oxidation number of −1or 0.

Note that the effects of the present invention are not obtained byisocyanic acid n-hexyl ester containing no sulfur atom, although theester is similar to the above isothiocyanic acid cyclohexyl ester.

(vi) Thioureas, isothioureas, dithiocarbamic acids, and salts and estersthereof.

More specifically, typical examples include those represented by thefollowing formulas.

where, R⁷, R⁸, R⁹, and R¹⁰ are the same or different, and are each ahydrogen atom or a hydrocarbon group having 1 to 40, more preferably 1to 20, and further preferably 1 to 10 total carbon atoms which may havea substituent.

Moreover, R⁷, R⁸, R⁹, and R¹⁰ may be bonded together to form amonocyclic or polycyclic ring (including fused rings).

More specifically, thioureas and dithiocarbamic acids as shown in thefollowing formulas can be exemplified.

Note that the sulfur atoms in these compounds have an oxidation numberof 0.

(vii) Substituted or unsubstituted thiophenes, or substituted orunsubstituted thiazoles.

More specifically, typical examples include those represented by thefollowing formula.

where A is a carbon atom or a nitrogen atom.

Moreover, R¹¹, R¹², and R¹³ (when A is a carbon atom), and R¹⁴ are thesame or different, and are each a hydrogen atom, a hydrocarbon grouphaving 1 to 40, more preferably 1 to 20, and further preferably 1 to 10total carbon atoms which may have a substituent, or an alkoxy grouphaving 1 to 40, more preferably 1 to 20, and further preferably 1 to 10total carbon atoms which may have a substituent.

However, when A is a nitrogen atom, R¹³ is an unshared electron pair ofthe nitrogen atom. When A is a carbon atom, R¹¹ and R¹³ may be bondedtogether to form a ring.

Further, R¹² and R¹⁴ may be bonded together to form an alicyclic ring ora benzene ring.

More specifically, thiophene and benzothiazole can be exemplified.

Note that the sulfur atom in each of these compounds has an oxidationnumber of 0.

Note that the effects of the present invention are not obtained bybenzoxazole having a structure equivalent to that of benzothiazoleexcept that a sulfur atom thereof is substituted with an oxygen atom.

(7) Examples of the inorganic sulfur compounds as the sulfur material(b) include dithionous acid (H₂S₂O₄) and salts thereof.

Examples of a counter cation in the salts include alkali metals,alkaline earth metals, ammonium, and primary to quaternary ammonium(such as pyridinium, guanidinium, tetrapropylammonium, pyrrolidinium,piperidinium, and the like).

Note that since a dithionite ion is assumed to have the followingstructure, the sulfur atoms in this compound have an oxidation number of+3.

More specifically, sodium dithionite can be exemplified.

FIG. 4A shows that when the catalyst electrode is surface-treated withthis sulfur material, the treated surface of the catalyst electrode hasa photoelectron peak within a binding energy range of 161 to 165 eV inan X-ray photoelectron spectrum.

(8) An example of the organic sulfur compounds as the sulfur material(b) includes a sulfone compound of the following general formula (XXIV).Sulfur in this compound has an oxidation number of +4.

where R¹⁵ and R¹⁶ each independently represent a hydrocarbon grouphaving 1 to 40, more preferably 1 to 20, further preferably 1 to 10, andstill further preferably 1 to 4 total carbon atoms which may have asubstituent. R¹⁵ and R¹⁶ may be bonded together to form a ring.

More specifically, sulfolane [the following formula (XXV)] can beexemplified.

As a typical example of the sulfone compound, FIG. 4 b shows that whenthe catalyst electrode is surface-treated with sulfolane, the treatedsurface of the catalyst electrode has a photoelectron peak within abinding energy range of 161 to 165 eV in an X-ray photoelectronspectrum.

(9) Note that even when the catalyst electrode is surface-treated usingdimethyl sulfoxide having an oxidation number of +2, sodium sulfite(Na₂SO₃) having an oxidation number of +4, or sodium sulfate (Na₂SO₄)having an oxidation number of +6 according to the procedure (1) i. ofthe platinum dissolution test in Examples, a photoelectron peak is notobserved within the binding energy range of 161 to 165 eV in the X-rayphotoelectron spectrum of the surface of the surface-treated catalystelectrode (see FIGS. 3B, 5A, and 5B), and the effects of the presentinvention are not obtained, either (see test numbers 16, 19, and 20 inTable 1). Presumably, these compounds are not reduced to sulfur in a lowoxidation state (having an oxidation number of −2 to 0) on the surfaceof the catalyst electrode; hence, the effects of the present inventionare not demonstrated.(10) In the present invention, the sulfur material (a) has a molecularweight of 32 to 10,000, but has a low molecular weight of preferably 32to 5,000, more preferably 32 to 1,000, and further preferably 32 to 500.Hence, π-conjugated conductive polymers and conductive binders such aspolypyrrole, polyaniline, and polythiophene as described in PTLs 1 and 4are excluded.

Generally, a polymer forms a three-dimensional film structure.Accordingly, it is thought that a polymer film formed on a catalystcompletely masks the highly active surface of the metal catalyst,blocking the contact between the catalyst and the reaction substrate,which may lower the catalyst performance. Meanwhile, alow-molecular-weight single molecular layer or single atomic layer isconsiderably thin, and accordingly does not block the contact betweenthe catalyst and the substrate; thus, it can be expected that theperformance thereof is not lowered.

Moreover, in the present invention, the molecular weight of the sulfurmaterial (b) is not particularly limited, but the sulfur material (b)has a low molecular weight of preferably 32 to 10,000, more preferably32 to 5,000, further preferably 32 to 1,000, and still furtherpreferably 32 to 500.

2) Catalyst electrode and Surface treatment with particular sulfurmaterial

(1) The catalyst electrode in the present invention has a catalyst layermade of a platinum group element formed on an electrode substrate.

The platinum group element is Ru, Rh, Pd, Os, Ir, or Pt, more preferablyPt.

The electrode substrate is not particularly limited, as long as it has acorrosion resistance to a corrosive component in an electrolyte layersealed between the catalyst electrode and a semiconductor electrode.Examples of the electrode substrate include metal materials such astitanium, nickel, and tungsten; conductive glass materials such as FTO(fluorine-doped tin oxide film), ITO (indium tin oxide film), and ATO(antimony tin oxide film); metal oxide materials such as zinc oxide andtitanium oxide; and the like. Above all, from the viewpoint of cost anddurability, metal materials and conductive glasses are preferably used.More preferable are titanium among metal materials and FTO amongconductive glass materials.

Additionally, examples of the platinum group catalyst layer includethose formed by: a plating method, a sputtering method, an applicationof a solution of chloroplatinic acid or the like, a printing method witha paste such as chloroplatinic acid, and other methods. Particularly,for use in a small test cell, one prepared by a sputtering methodcapable of forming a uniform film is preferably used. In a case of alarge module requiring patterning such as a submodule, a film ispreferably formed by a screen printing method.

(2) Examples of the surface treatment on the catalyst electrode with thesulfur material include (i) treatments (such as coating, immersing, orexposure to a gaseous sulfur material in a case where the sulfurmaterial is in a gaseous form) on the catalyst electrode with a preparedsolution of the sulfur material, and (ii) a method in which when thecatalyst electrode layer is prepared on the catalyst electrode, asubstrate is treated with the sulfur material together with the catalystor a catalyst precursor so that the sulfur material can also beincorporated into the catalyst electrode layer.

More specifically, as an example of the former method (i), the electrodehaving the catalyst layer formed is immersed for a certain period in asolution containing the sulfur material dissolved (if necessary,subjected to heating), and then the electrode is taken out, washed witha solvent and dried.

As the latter method (ii), more specifically, the following isconceivable: a printing paste in which a platinum group elementprecursor is mixed with the sulfur material is prepared, printed andsintered on the electrode substrate, and as necessary, subjected to areduction treatment, or the sulfur material (for example, hydrogensulfide gas) is allowed to react simultaneously with platinum filmformation by a CVD method, thereby forming a catalyst layer containingthe sulfur material.

Note that, regarding the conditions of such surface treatments, theprocesses in the procedure (i) i. of the platinum dissolution test inExamples serve as the standard process requirements. The type of asolvent used here is determined according to the following criteria.

a) Basically, water is used as the solvent.

b) If a solution of a predetermined concentration cannot be prepared dueto poor water solubility, 3-methoxypropionitrile is used as the solvent.

c) Furthermore, benzene is used for such poorly soluble substances suchas elemental sulfur and carbon disulfide, from which a solution of apredetermined concentration cannot be prepared even using water and3-methoxypropionitrile.

According to the above criteria, generally, it is assumed that water isused as the solvent for the inorganic sulfur materials, while3-methoxypropionitrile is used as the solvent for the organic sulfurmaterials.

Nevertheless, it is considered that the difference in the type ofsolvent hardly has any influence. Accordingly, even if a solution of apredetermined concentration cannot be prepared using the three types ofsolvent by any chance, a solvent having the highest possible solubilityand having a relatively high boiling point (higher than 85° C.) ispreferably selected from among readily available solvents for use fromthe viewpoint of handling easiness.

3) Electrolyte layer

The electrolyte layer of the present invention comprises chemicalspecies as a redox couple, a solvent of an electrolyte solution fordissolving these, and an additive as a desired component.

Examples of the chemical species as a redox couple include I₂/I₃ ⁻ basedor Br₂/Br₃ ⁻ based redox system, and the like. An I₂/I₃ ⁻ based redoxsystem is preferable. Examples of a counterion thereof include lithiumsalts, imidazolium salts, other quaternary ammonium salts, and the like.Above all, from the viewpoint of achieving both high performances andexcellent durability, imidazolium salts or other quaternary ammoniumsalts are more preferably used.

As the solvent of an electrolyte solution, it is possible to use any ofnon-aqueous organic solvents, ambient temperature molten salts, water,protic organic solvents, and the like, as long as electrolytes such asthe chemical species as a redox couple can be dissolved sufficiently.non-aqueous organic solvents are preferable examples. Above all,3-methoxypropionitrile is particularly preferable from the viewpoint ofachieving both high performances and excellent durability.

Additionally, the electrolyte solution has a viscosity of preferably0.35 mPa·s (20° C.) to 695 cPa·s (20° C.), more preferably 0.1 cPa·s(20° C.) to 10 cPa·s (20° C.)

4) Photo-sensitizing dye

(1) As the photo-sensitizing dye, various metal complexes and organicdyes having an absorption in the visible region and/or the infraredregion can be used and are adsorbed to the metal oxide semiconductorfilm by any known methods, for example, a method in which a thin film ofan oxide semiconductor such as titanium dioxide is immersed in a dyesolution at a predetermined temperature (a dipping method, a rollermethod, an air knife method, or the like), or a method in which a dyesolution is applied to a surface of an oxide semiconductor layer (a wirebar method, an application method, a spin method, a spray method, anoffset printing method, a screen printing method, or the like.

(2) Note that in the present invention, particularly the effects of thepresent invention are considerably demonstrated especially when a dyeused as the photo-sensitizing dye is not a metal complex; or even when ametal complex dye is used as the photo-sensitizing dye, if the metalcomplex dye thus used does not contain a ligand (thiocyanate ligand orthe like) containing a sulfur atom having an oxidation number of −2 to0.

The reason is as follows. When a metal complex dye is used as thephoto-sensitizing dye, if the electrolyte solution contains iodideanions, tert-butylpyridine, or the like, an exchange reaction takesplace with the aforementioned ligand of the metal complex. This providesa possibility that the ligand of the metal complex is released into theelectrolyte solution. Hence, if a ligand such as a thiocyanate ligandcontaining a sulfur atom having an oxidation number of −2 to 0 is usedas such a ligand, it may contribute to formation of a sulfur coated filmon a catalyst electrode to some extent in a mode as described in asecond embodiment of the present invention below.

5) As other components of the dye-sensitized solar cell of the presentembodiment, those normally used can be used suitably.

(1) Semiconductor Electrode

The semiconductor electrode of the present invention is preferably atranslucent electrode and is made of an oxide semiconductor film formedon a transparent conductive substrate. The oxide semiconductor layersupports the spectrally photo-sensitizing dye.

As the oxide semiconductor, it is possible to use known porous materialssuch as titanium oxide, zinc oxide, tin oxide, tin-doped indium oxide,zirconium oxide, and magnesium oxide, which can be formed on thetransparent conductive substrate by a spin coating method, a spraymethod, a dipping method, a screen printing method, a doctor blademethod, an ink-jet method, or the like. A spin coating method, a spraymethod or a dipping method is preferably employed from the viewpoint ofeasiness of the operation, while a screen printing method is preferablyemployed from the viewpoint of mass production.

(2) Transparent Conductive Substrate

As the transparent conductive substrate, it is preferable to use thosehaving, as a transparent conductive film, a film of titanium oxide, zincoxide (optionally doped with antimony or aluminium), indium oxide(optionally doped with tin or zinc), tin oxide [optionally doped withantimony (ATO), or optionally doped with fluorine (FTO)], or the likeformed on a transparent substrate such as a transparent glass or atransparent resin film.

(3) Method for Producing Dye-Sensitized Type Solar Cell

The dye-sensitized type solar cell is produced by bonding thesemiconductor electrode and the catalyst electrode together with asealant in between.

For example, a partition wall of the sealant is formed on thetransparent conductive substrate having the semiconductor electrode. Thepartition wall can be formed easily by using a printing technique suchas screen printing. The sealant is not particularly limited, as long asit has a corrosion resistance to a corrosive component in theelectrolyte. Examples of the sealant include thermoplastic resins,thermosetting resins, ultraviolet curable resins, electron beam curableresins, metals, rubbers, and the like. At least the surface of thesealant needs to be electrically insulative. If the sealant iselectrically conductive, the surface thereof is covered by anelectrically insulating material such as various resins and rubbers.

Then, the semiconductor electrode and the catalyst electrode are bondedtogether with the sealant in between. In this event, attention should bepaid in order that the two electrodes may be arranged parallel to eachother by uniform application of a pressure.

Further, the partition wall of the sealant keeps a uniform space betweenthe semiconductor electrode and the catalyst electrode, in which theelectrolyte is encapsulated. Thus, a dye-sensitized solar cell isproduced.

Note that the photo-sensitizing dye can be fixed on the semiconductorelectrode by, for example, immersing the semiconductor electrode into adye solution before or after the bonding step.

2. Second Embodiment of the Present Invention

In the second embodiment of the present invention, a dye-sensitized typesolar cell is characterized in that an electrolyte layer contains thefollowing particular sulfur material, that is,

(a) at least one sulfur material having a molecular weight of 32 to10,000 selected from elemental sulfur, inorganic sulfur compounds eachcontaining at least one sulfur atom having an oxidation number of −2 to0, and organic sulfur compounds each containing at least one sulfur atomhaving an oxidation number of −2 to 0;

(b) at least one sulfur material containing no sulfur atom having anoxidation number of −2 to 0 selected from inorganic sulfur compoundseach containing at least one sulfur atom having an oxidation number of+1 to +4 and organic sulfur compounds each containing at least onesulfur atom having an oxidation number of +1 to +4; or

(c) a mixture of the sulfur materials (a) and (b).

Nevertheless, it is required that the sulfur material (b) is such amaterial that when the catalyst electrode is surface-treated with thesulfur material (b), the treated surface of the catalyst electrode has aphotoelectron peak within a binding energy range of 161 to 165 eV in anX-ray photoelectron spectrum.

Moreover, when the sulfur material (a) is a guanidinium thiocyanate, theguanidinium thiocyanate is required to have a concentration of less than0.1 M in the electrolyte layer, and

when the sulfur material (a) is 1-ethyl-3-methylimidazolium thiocyanate,the 1-ethyl-3-methylimidazolium thiocyanate is required to be less than35% by volume in a solvent of an electrolyte solution.

In the present embodiment, as in the first embodiment of the presentinvention, the effect of suppressing dissolution of a catalyst and atleast in a preferred mode, the effect of improving the catalyticactivity can be obtained as well. Conceivably, these effects are derivedfrom the sulfur material added into the electrolyte layer, the sulfurmaterial forming a sulfur coated film on a catalyst layer. In otherwords, in the first embodiment of the present invention, a catalystcounter electrode may be brought into contact with a sulfur material inadvance to form a sulfur coated film directly on a catalyst layer, whilein the present embodiment, a sulfur coated film can be also formed on acatalyst layer as time elapses by incorporating a sulfur material intoan electrolyte layer.

Further, in the case of the present embodiment, the sulfur material inthe electrolyte continues to be in contact with the catalyst surface inthe cell all the times. Accordingly, it can be expected that the sulfurcoated film lasts long.

Note that among modes in which an electrolyte layer contains a sulfurmaterial, a mode in which the sulfur material is present as an additivein a dissolved state in the solvent of the electrolyte solution istypical, but is not always limited thereto, as long as the effects ofthe present invention are not impaired. For example, it is conceivablethat a sulfur material containing a sulfur atom having an oxidationnumber of +4 such as sulfolane may be used as a solvent of anelectrolyte solution, while some of the inorganic sulfur materials maybe used as an ionic liquid in a solvent of an electrolyte solution.

1) In the dye-sensitized solar cell of the present embodiment, unlikethe first embodiment of the present invention, the catalyst electrodedoes not always have to be surface-treated with a particular sulfurmaterial, but the electrolyte layer always contains a particular sulfurmaterial. Moreover, there are other differences in consideration of NPLs2, 3, and 5: when the sulfur material (a) as the particular sulfurmaterial is guanidinium thiocyanate, the guanidinium thiocyanate has aconcentration of less than 0.1 M in the electrolyte layer; and when thesulfur material (a) is 1-ethyl-3-methylimidazolium thiocyanate, the1-ethyl-3-methylimidazolium thiocyanate is less than 35% by volume inthe electrolyte layer.

Nonetheless, as to points other than such differences, basically thedescription of the first embodiment of the present invention is appliedappropriately to the dye-sensitized type solar cell of the presentembodiment as well.

2) Particular sulfur material in the electrolyte layer

(1) The particular sulfur material of the present embodiment isbasically the same particular sulfur material used in the firstembodiment of the present invention.

Nonetheless, NPLs 2 and 3 disclose an example where the minimum amountof guanidinium thiocyanate used in an electrolyte solution of adye-sensitized solar cell is 0.1 M. NPL 5 discloses an example where1-ethyl-3-methylimidazolium thiocyanate is used as a solvent of anelectrolyte solution, which is a cosolvent with1-propyl-3-methylimidazolium iodide at a volume ratio of 7:13(1-ethyl-3-methylimidazolium thiocyanate:1-propyl-3-methylimidazoliumiodide) in an electrolyte solution of a dye-sensitized solar cell.Hence, the particular sulfur material of the present embodiment differsfrom the first embodiment of the present invention in that: whenguanidinium thiocyanate is used as the sulfur material, the guanidiniumthiocyanate has a molar concentration of less than 0.1 M, preferably0.05 M or less, in the electrolyte layers; in addition, when1-ethyl-3-methylimidazolium thiocyanate is used as the sulfur material,the 1-ethyl-3-methylimidazolium thiocyanate has a volume percent of lessthan 35% by volume, preferably 20% by volume or less, more preferably10% by volume or less, and further preferably 5% by volume or less, inthe solvent of the electrolyte solution.

Note that the “volume percent” of the 1-ethyl-3-methylimidazoliumthiocyanate in the solvent of the electrolyte solution is calculated asfollows. Specifically, the volume percent of the1-ethyl-3-methylimidazolium thiocyanate is calculated based on a sum ofvolumes of a main solvent and one or more cosolvents. The main solventis a liquid component present in the largest amount (based on the volumeof the component present alone) among components in a liquid state atstandard ambient temperature and pressure (25° C., 1 atm) of theelectrolyte layer. The one or more cosolvents are1-ethyl-3-methylimidazolium thiocyanate (ionic liquid) and one or moreoptional liquid components, each of the optional liquid component havinga one-fifth volume or more of the main solvent (based on the volumes ofthe components each present alone).

Nonetheless, NPL 2 merely states that guanidinium cations added into anelectrolyte solution suppress charge recombination and shift the bandedge of titania to thereby improve the open-circuit voltage Voc, butdoes not mention the effect of thiocyanate anion. Moreover, NPL 3 alsoreports that: guanidinium thiocyanate positively shifts the flatbandpotential of titania to thereby increase the electron injectionefficiency and improve the short-circuit current density Jsc; andguanidinium cation chemisorbed on titania passivates the recombinationsites on titania. However, NPL 3 does not mention the effect ofthiocyanate anion itself. Further, NPL 5 does not necessarily describethe effect of preventing elution of a catalyst from a catalyst electrodeparticularly by 1-ethyl-3-methylimidazolium thiocyanate.

Meanwhile, the sulfur material of the present embodiment is preferably asulfur material which is not only simply distributed uniformly andundynamically in the electrolyte layer but is likely to diffusedynamically. More specifically, the sulfur material is preferablydispersed, further preferably dissolved, in the electrolyte.

(2) The concentration of the sulfur material in the electrolyte layer ispreferably a concentration exceeding at least 0.001 M to obtain theeffects of the present invention. If the concentration is 0.01 M orhigher, more preferable sufficient effects can be obtained. In addition,a preferable upper limit of the concentration is within the range notimpairing the effects of the present invention, and the concentration atthe solubility level or lower is preferable.

3. Third Embodiment of the Present Invention

(1) The present embodiment basically corresponds to a combinationaldye-sensitized solar cell of the first and the second embodiments of thepresent invention. Accordingly, basically, the descriptions of the firstand the second embodiments of the present invention in 1. and 2. abovecan be applied here.

(2) However, unlike the second embodiment, when a second sulfur material(a2) of the present embodiment is guanidinium thiocyanate, theguanidinium thiocyanate does not always have to have a concentration ofless than 0.1 M in an electrolyte layer; moreover, when the secondsulfur material (a2) of the present embodiment is1-ethyl-3-methylimidazolium thiocyanate, the 1-ethyl-3-methylimidazoliumthiocyanate does not always have to be less than 35% by volume in theelectrolyte layer. This is because NPLs 2, 3, and 5 do not particularlyexplicitly describe this combinational embodiment of the first and thesecond embodiments of the present invention.

(3) In addition, in the present embodiment, it is not always necessaryto use the same material for: the second sulfur material contained as aparticular sulfur material in the electrolyte layer, and a first sulfurmaterial used as a particular sulfur material for the surface treatmenton a catalyst electrode.

Moreover, when the second sulfur material used as an additive in theelectrolyte layer is guanidinium thiocyanate, the first sulfur materialother than guanidinium thiocyanate can be used in combination as thefirst sulfur material. NPLs 2, 3, and 5 do not describe or suggest suchan embodiment at all.

Further, when the second sulfur material used as a solvent of anelectrolyte solution in the electrolyte layer is1-ethyl-3-methylimidazolium thiocyanate, the first sulfur material otherthan 1-ethyl-3-methylimidazolium thiocyanate can be used in combinationas the first sulfur material. NPLs 2, 3, and 5 do not describe orsuggest such an embodiment at all.

(4) The present embodiment is preferable in that the effects of thepresent invention can be further effectively demonstrated in comparisonwith other embodiments of the present invention: the first and thesecond embodiments.

Specifically, in the case of the second embodiment of the presentinvention alone, it is considered to take some time to form the sulfurcoated film on the catalyst layer. Nevertheless, if the first embodimentof the present invention is employed in combination, the effect ofsuppressing dissolution of a catalyst and at least in a preferred mode,the effect of improving the catalytic activity can be obtainedimmediately after a cell is produced.

Moreover, in comparison with the first embodiment of the presentinvention, it is also possible to complement the dissolution-suppressingeffect with a long-lasting improvement of the sulfur coated film, andthe effect of improving the catalytic activity at least in a preferredmode with a long-lasting effect thereof, which are advantages of thepresent embodiment.

Further, the present embodiment has an advantage that more sulfurmaterials are allowed to exist (total of those on the surface of thecatalyst electrode and in the electrolyte layer) than in the first andthe second embodiments of the present invention.

4. Fourth Embodiment of the Present Invention

(1) The present embodiment generalizes the dye-sensitized solar cells ofthe first to the third embodiments of the present invention as a methodfor preventing elution of a platinum group catalyst.

Thus, basically, the descriptions of the first to the third embodimentsof the present invention in the sections 1., 2., and 3. above can beapplied here.

(2) Nonetheless, in the present embodiment specifying the method forpreventing elution of a platinum group catalyst, when an electrolytelayer contains the above-described particular sulfur material, unlikethe second embodiment of the present invention, if a second sulfurmaterial (a2) is guanidinium thiocyanate in the present embodiment, theguanidinium thiocyanate does not always have to have a concentration ofless than 0.1 M in the electrolyte layer; moreover, when the secondsulfur material (a2) is 1-ethyl-3-methylimidazolium thiocyanate in thepresent embodiment, the 1-ethyl-3-methylimidazolium thiocyanate does notalways have to be less than 35% by volume in the electrolyte layer. Thisis because NPLs 2, 3, and 5 above neither describe nor suggest at allthe effect of preventing elution of a platinum group catalyst by usingguanidinium thiocyanate or 1-ethyl-3-methylimidazolium thiocyanate in anelectrolyte layer.

(3) In the present embodiment, when a first sulfur material and a secondsulfur material are used in combination, it is not always necessary touse the same material for: the second sulfur material contained as aparticular sulfur material in the electrolyte layer, and the firstsulfur material used as a particular sulfur material for the surfacetreatment on a catalyst electrode.

EXAMPLES Platinum Dissolution Test

(1) According to the following procedures i. to vi., titanium plates onwhich platinum was vapor-deposited were treated with various sulfurmaterials. Then, percentages of retained platinum were measured. Table 1shows the result.

i. A titanium plate on which 10-nm platinum was vapor-deposited bysputtering was immersed into a solution in which a sulfur material wasdissolved in a solvent (concentration: 0.1 M), followed by heating at85° C. (70° C. in a case where benzene was used as the solvent) for 1hour, and by washing with the solvent containing no sulfur material toprepare a test piece.

ii. Platinum of the test piece was quantified using an energy dispersiveX-ray fluorescence spectrometer (EDX900 manufactured by ShimadzuCorporation). The La radiation intensity (9.44 keV) of platinum at thistime was designated as I_(Pt0).

iii. The test piece was immersed into an electrolyte solution (preparedby dissolving 0.1 M iodine, 0.8 M 1-propyl-3-methylimidazolium iodide,and 0.3M tert-butylpyridine in 3-methoxypropionitrile), and placed andkept in a drier at 85° C. for 16 hours.

iv. The immersed test piece was taken out from the drier. Using ethanol,the test piece was washed and dried.

v. Again, platinum was quantified by X-ray fluorescence analysis. The Laradiation intensity (9.44 keV) of platinum at this time was designatedas I_(Pt1).

vi. The percentage of retained platinum was calculated according to thefollowing formula. Those having the percentage of retained platinum of90% or higher, 20% or higher but lower than 90%, and lower than 20% wererespectively evaluated as “A”, “B”, and “C”. Nevertheless, no test pieceamong test numbers 1 to 23 listed in Table 1 was evaluated as “B”.

Percentage of retained platinum (%)=(I_(Pt1)/I_(Pt0))×100

TABLE 1 Oxidation number of Treatment Percentage Test sulfur atom intemperature of retained number Sulfur material sulfur material Solvent(° C.) platinum (%) 1 sodium sulfide −2 water 85 A 2 1-dodecanethiol −13-methoxy- 85 A propionitrile 3 2,4,6-trimercapto- −1 water 85 As-triazine mono sodium salt 4 guanidinium 0 water 85 A thiocyanate 5potassium 0 water 85 A thiocyanate 6 cyclohexyl 0 3-methoxy- 85 Aisothiocyanate propionitrile 7 rhodanine 0, 0 3-methoxy- 85 Apropionitrile 8 ethyl rhodanine 0, 0 3-methoxy- 85 A propionitrile 9thiourea 0 water 85 A 10 benzothiazole 0 3-methoxy- 85 A propionitrile11 methionine 0 water 85 A 12 thiophene 0 water 85 A 13 sulfur 0 benzene70 A 14 carbon disulfide 0 benzene 70 A 15 sodium 0, +4 water 85 Athiosulfate 16 dimethyl +2 3-methoxy- 85 C sulfoxide propionitrile 17sodium dithionite +3 water 85 A 18 sulfolane +4 water 85 A 19 sodiumsulfite +4 water 85 C 20 sodium sulfate +6 water 85 C 21 benzoxazole —3-methoxy- 85 C propionitrile 22 n-hexyl — 3-methoxy- 85 C isocyanatepropionitrile 23 No pretreatment — — — C

Note that at the stage of the procedure ii., that is, at the stageimmediately after the pretreatment with the sulfur material, the testpieces were subjected to X-ray photoelectron spectrometry (S2p region,with AXIS-His manufactured by KRATOS ANALYTICAL).

As a result, in the test numbers 1, 5, 9, 13, 15, 17, and 18, aphotoelectron peak was observed within a binding energy range of 161 to165 eV, which indicates that a sulfur coated film was formed onplatinum. On the other hand, in the test pieces of the test numbers 16,19, and 20, a photoelectron peak was not observed within the range of161 to 165 eV. Nevertheless, in the test numbers 16 and 19, aphotoelectron peak was observed within a range of 165 to 169 eV.

Normally, in an X-ray photoelectron spectrometry, the higher theoxidation state of an atom, the more the binding energy shifts to thehigh energy side. Taking this into consideration, it is assumed that thepeak within 161 to 165 eV indicates sulfur in a low oxidation state(thiolate species, thiol species, or sulfide species having an oxidationnumber of −2 to 0), and that the peak within 165 to 169 eV indicatessulfur in a higher oxidation state.

Presumably, the fact that the test numbers 1, 5, 9, 13, 15, 17, and 18demonstrated a high percentage of retained platinum indicates thatsulfur in a low oxidation state (thiolate species, thiol species, orsulfide species having an oxidation number of −2 to 0) adsorbed onplatinum suppresses elution of the platinum catalyst in the electrolytesolution.

[Dye-Sensitized Solar Cell Experiment]

(1) According to the following procedures, dye-sensitized solar cellswere produced.

i. A titanium oxide paste [PST-21NR manufactured by Catalysts &Chemicals Industries Co., Ltd.] was screen-printed to a film thicknessof 8 μm on a squared area having a side of 1 cm of a substrate (glassplate with a fluorine-doped tin oxide film, 30 mm×25 mm). After drying,a titanium oxide paste [PST-400C manufactured by Catalysts & ChemicalsIndustries Co., Ltd.] was further screen-printed thereon to a filmthickness of 4 μm. By sintering this at 500° C., a power generatinglayer was formed.

ii. An electrode having the power generating layer formed was immersedinto a dye solution [dye: MK-2 manufactured by Soken Chemical &Engineering Co., Ltd., concentration: 0.45 M, solvent: toluene] at 40°C. for 3 hours to thereby support the dye on titanium oxide of the powergenerating layer. Thus, an anode electrode was obtained.

Note that the dye MK-2 is not a metal complex but a dye having thefollowing structure.

Note that although this dye also contains a sulfur atom having anoxidation number of 0, the dye is a non-complex dye and hardlyinfluences the effects of the invention of the present application,unlike a system such a metal complex dye by which ligand exchangereadily occurs.

iii. A hot melt adhesive was applied around the power generating layerof the anode electrode. This anode electrode was adhered to aseparately-prepared 10-nm platinum-sputtered titanium plate (cathodeelectrode) having an electrolyte solution injection hole with theadhesive in such a manner that the two electrodes are arranged parallelto each other with a uniform space of approximately 50 μm in between.

Here, the 10-nm platinum-sputtered titanium plate used in Examples hadbeen immersed in an aqueous solution containing 0.1 M sulfur material,heated at 85° C. for 1 hour, then washed with water and dried. The 10-nmplatinum-sputtered titanium plate used in Comparative Example had notbeen subjected to such a sulfur material treatment.

iv. Then, an electrolyte solution was injected through the electrolytesolution injection hole. The electrolyte solution used here was thefollowing electrolyte solution A or electrolyte solution B.

Electrolyte solution A: a solution of 0.1 M iodine, 0.8 M1-propyl-3-methylimidazolium iodide, and 0.5 M 1-methylbenzimidazole in3-methoxypropionitrile as a solvent.

Electrolyte solution B: a solution of 0.1 M iodine, 0.8 M1-propyl-3-methylimidazolium iodide, 0.5 M 1-methylbenzimidazole, and0.01 M cyclohexyl isothiocyanate with 3-methoxypropionitrile as asolvent.

v. The electrolyte solution injection hole was sealed with an adhesive,and a solder for terminal formation was applied onto the anodeelectrode. Thus, an experimental cell was completed.

(2) The performances, photoelectric conversion efficiency and seriesresistance, of each dye-sensitized solar cell obtained as describedabove were evaluated using a solar simulator (YSS-200 manufactured byYamashita Denso Corporation, AM 1.5, 1 SUN (100 mW/cm²), and the resultas shown in the following table was obtained.

TABLE 2 photoelectric percentage of Sulfur initial initial conversionseries conversion material photoelectric series efficiency resistanceefficiency for pre- conversion resistance*2, after 4 after 4maintained*3 treatment*1 Electrolyte efficiency*2 *4 days*2 days*2, *4(%) Example 1 potassium A 5.86 7.6 5.07 7.9 87 thiocyanate Example 2sodium A 5.82 7.5 4.92 8.0 85 sulfide Example 3 thiourea A 5.90 7.5 5.097.9 86 Example 4 — B 5.45 10.1 4.84 8.3 89 Example 5 thiourea B 5.79 7.55.08 7.7 88 Comparative — A 5.45 10.2 1.58 68.3 29 Example *1See Table 1for the process condition of each sulfur material for pretreatment.*2The performances measured immediately after completion of theexperimental cell were initial photoelectric conversion efficiency andinitial series resistance. After these measurements, the experimentalcell was further stored in a drier at 85° C. for 4 days. After that, thecell was taken out and cooled to room temperature. Then, theperformances measured were photoelectric conversion efficiency after 4days and series resistance after 4 days. *3The percentage of conversionefficiency maintained in Table 2 was obtained according to the followingformula: Percentage of conversion efficiency maintained (%) = 100 ×[(photoelectric conversion efficiency after 4 days)/(initialphotoelectric conversion efficiency)]. Moreover, the photoelectricconversion efficiency was calculated according to the following formula:Photoelectric conversion efficiency (%) = 100 × [(short-circuit currentdensity × open-circuit voltage × fill factor)/(irradiated solar lightenergy)]. *4: The series resistance here is calculated according to thefollowing formula: Series resistance (Ω) = (−1)/K, where K in theformula represents a slope of a tangent line at a point crossing thevoltage axis (open-circuit voltage point) of a current-voltage curve forthe solar cell. The series resistance thus obtained represents theseries component of the internal resistance on the catalyst, when thecatalyst performance is high, the series resistance shows a relativelylow value.

As apparent from Table 2 above, in a case as in Examples 1, 2, 3, and 5where the catalyst electrode used was pretreated with a sulfur material,the catalytic activity was improved, and the initial series resistancewas decreased. As a result, a high initial conversion efficiency wasobtained. Further, after 4 days elapsed at 85° C. also, no largeincrease in the series resistance was observed. It can be understoodthat dissolution of the catalyst platinum was suppressed.

Meanwhile, in a case as in Example 4 where no pretreatment with a sulfurmaterial was performed, this resulted in a high resistance initially incomparison with Examples 1, 2, 3, and 5; however, as 4 days elapsed, theresistance value was decreased. This decrease in the resistance valueindicates not only that the catalyst platinum was not dissolved, butalso presumably the sulfur material (cyclohexyl isothiocyanate) addedinto the electrolyte solution B formed a sulfur coated film in a lowoxidation state (having an oxidation number of −2 to 0) on the catalystplatinum and improved the catalytic activity.

As described above, it is clear that the present technique is essentialfor dye-sensitized solar cells to obtain a practical durability.

The invention claimed is:
 1. A dye-sensitized solar cell wherein thedye-sensitized solar cell is a dye-sensitized type solar cellcomprising: a semiconductor electrode containing a photo-sensitizingdye; an electrolyte layer containing chemical species as a redox couple;and a counter electrode disposed opposite to the semiconductor electrodewith the electrolyte layer interposed therebetween, the counterelectrode is a catalyst electrode containing a platinum group, theelectrolyte layer contains the chemical species as the redox couple, asolvent of an electrolyte solution for dissolving these, and anadditive, and the additive comprises any one of sulfur materials in adissolved state in the electrolyte layer: (a) at least one sulfurmaterial having a molecular weight of 32 to 10,000 selected fromelemental sulfur, inorganic sulfur compounds each containing at leastone sulfur atom having an oxidation number of −2 to 0, and organicsulfur compounds each containing at least one sulfur atom having anoxidation number of −2 to 0; (b) at least one sulfur material containingno sulfur atom having an oxidation number of −2 to 0 selected frominorganic sulfur compounds each containing at least one sulfur atomhaving an oxidation number of +1 to +4 and organic sulfur compounds eachcontaining at least one sulfur atom having an oxidation number of +1 to+4; and (c) a mixture of the sulfur materials (a) and (b), with theproviso that the sulfur material (b) is such a material that when thecatalyst electrode is surface-treated with the sulfur material (b), thetreated surface of the catalyst electrode has a photoelectron peakwithin a binding energy range of 161 to 165 eV in an X-ray photoelectronspectrum, when the sulfur material (a) is guanidinium thiocyanate, aconcentration of the guanidinium thiocyanate is less than 0.1 M in theelectrolyte layer, and when the sulfur material (a) is1-ethyl-3-methylimidazolium thiocyanate, a volume percent of the1-ethyl-3-methylimidazolium thiocyanate is less than 35% by volume inthe solvent of the electrolyte solution, and where the volume percent ofthe 1-ethyl-3-methylimidazolium thiocyanate is calculated based on a sumof volumes of a main solvent and one or more cosolvents where the mainsolvent is a liquid component present in a largest amount (based on thevolume of the component present alone) among components in a liquidstate at standard ambient temperature and pressure (25° C., 1 atm) ofthe electrolyte layer, and the one or more cosolvents are1-ethyl-3-methylimidazolium thiocyanate (ionic liquid) and one or moreoptional liquid components, each of the optional liquid componentshaving a one-fifth volume or more of the main solvent (based on thevolumes of the components each present alone).
 2. The dye-sensitizedtype solar cell according to claim 1, wherein the chemical species inthe electrolyte layer are an I₂/I₃ ⁻ based redox couple.
 3. Thedye-sensitized type solar cell according to claim 1, wherein thephoto-sensitizing dye is: a dye which is not a metal complex dye, or ametal complex dye which does not have a ligand containing a sulfur atomhaving an oxidation number of −2 to 0.